An Infrared GLIMPSE into Astronomy's 'Zone of Avoidance'

Written by Linda Vu
December 20, 2005

This nine-degree swath of sky imaged by Spitzer is the first part of the GLIMPSE project released.NASA/JPL-Caltech/E. Churchwell (Univ. Wisconsin)

Astronomy's "zone of avoidance" may sound like a place where astronomers send their misbehaved kids for a "timeout," but actually it is a phrase used by many scientists to describe our dusty Milky Way galaxy.

As inhabitants of a flat galactic disk and spiral arm, Earth and its solar system have an inside-out and edge-on view of their host galaxy. Thus, most of the galaxy is condensed into a blurry narrow band of light that stretches completely around the sky, also known as the galactic plane. From Earth's perspective, this plane is further obscured by thick clouds of dust and gas that hover around the galaxy's center and make up its spiral arms.

Like a bright metropolis smothered in a thick fog, this heavy cosmic haze blocks billions of Milky Way stars and structures from optical view. Hence, many astronomers refer the galactic plane as the "zone of avoidance," and purposely avoid pointing visible-light and ultraviolet telescopes at it.

However, by using the Spitzer Space Telescope's dust-piercing Infrared Array Camera (IRAC) to lift the veil of galactic fog, astronomers on the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) team are showing the world that the "zone of avoidance" no longer needs to be avoided. In the infrared, the Milky Way offers great insights into the processes of star formation and galaxy evolution.

"By making the galactic plane transparent, Spitzer opens a new door for astronomers to study the Milky Way. With Spitzer, we can see stars in the plane and have even detected galaxies on the other side," said GLIMPSE Principal Investigator, Dr. Edward Churchwell.

Although GLIMPSE specifically counts among its primary goals mapping the Milky Way's structure, studying the properties of star formation, and observing the galaxy's distribution of massive and non-massive stars, scientists on the team hope for other unexpected discoveries

"When you finally get to look at a region of the sky that has previously been obscured by dust you don't really know what you're going to find and that's exciting," said GLIMPSE scientist, Dr. Robert Benjamin. "There is only one galaxy that we can study in this much detail and that is our own."

"Some of the most interesting science likely to come out of this project will be serendipitous discoveries, which will open up entirely new avenues of inquiry," adds Churchwell.

Finding the Milky Way

Spitzer has an advantage in looking at our Milky Way because infrared light has long wavelengths. As light waves travel throughout the universe they encounter "road blocks" in the form of gas and dust molecules. If the dust molecule is larger than the light's wavelength, then the light will be absorbed by dust. However, if the light's wavelength is longer, it may be able to jump over or move around the dust. Thus, although stars' ultraviolet and visible-light wavelengths are blocked by galactic clouds of dust, longer infrared wavelengths get through.

"This is the same phenomenon that makes the Sun seem red as it sets on Earth's horizon," said Benjamin. "Because there is more dust in your line of view as the Sun is setting, you only see the longer red and orange visible-light wavelengths."

According to Benjamin, by penetrating these dust clouds Spitzer provides a unique opportunity to study the otherwise hidden stars in our galaxy.

"Spitzer provides one of the clearest pictures of the Milky Way's stellar population that we are ever likely to get," said Benjamin. "Future larger and more sensitive telescopes are unlikely to be used for mapping projects, and telescopes that use longer wavelengths will mostly see cold dust rather than stars."

Covering approximately 220 square degrees of sky (equivalent to 440 full moons lined up side by side), GLIMPSE is one of the largest surveys made by Spitzer.

GLIMPSE Science: Stars

In a galaxy, stars are the most visible features to our eyes, and perhaps the most significant from a human perspective. They are essential to the creation of planets, life, and serve as the backbone of a galaxy's frame. Yet, there is much about the evolution of stars that remains a mystery.

According to Churchwell, when stargazers tilt their head up to view the night sky, they see a "hodgepodge of things."

"The 'stars' that you see all have different masses, are made up of different elements, sit at different distances in the Milky Way, and some are not actually stars at all, but galaxies outside of our own," said Churchwell. "However in a star cluster this isn't the case, and by observing star clusters in the Milky Way astronomers can learn a lot about stars that they can't find out any other way."

Astronomers know that in a cluster, all the stars are the same age, live at the same distance, and are made up of relatively the same element abundances, because they all form from the same molecular cloud of gas and dust. Thus, astronomers can see how mass affects the evolution of stars by studying how stars with different masses act in a cluster.

By studying the evolution of stars GLIMPSE astronomers can also gather information about the evolutional state of our Milky Way.

"In older galaxies we typically see very little star formation because most of the galaxy's materials (gas, dust, etc.) get caught up in stars," says Churchwell. "Because we can still see dust light up and being pushed around by stellar outflows in the galactic plane we know that the Milky Way is actively forming stars."

The GLIMPSE team also has an advantage in studying the life and death cycles of stars because Spitzer's sensitive infrared eyes allow astronomers to detect individual stars.

According to GLIMPSE Scientist Dr. Barbra Whitney, stars in the Milky Way are like trees in a forest. There are many different types of stars and depending on their compositions, they evolve differently.

"When stars near the end of their lives they eject different types of shells," says Whitney. "Stars that form oxygen shells will evolve much differently than stars that form carbon rich shells ... by understanding the end stages of so many other stars, we will have a better idea of the fate of our Sun 5 billion years from now."

Thus far, the team has catalogued more than 30 million stars in the inner Milky Way, and expects to identify more than 50 million stars by the end of the project.

This artist's rendering illustrates the observing ranges of GLIMPSE as it might appear if viewed from above our Milky Way galaxy. In this rendering, green represents the area captured in the GLIMPSE observations and the yellow dot indicated the location of our solar system. The red slice represents the 9 degrees of sky portrayed in the first panoramic image release from GLIMPSE.NASA/JPL-Caltech/R. Hurt (SSC)

Galactic Structure

Although the Milky Way is technically our galactic backyard, because of Earth's position inside the galaxy's flat disk, astronomers know more about the structure of galaxies billions of light-years away than they know about our own galaxy.

"Mapping the structure of our Milky Way galaxy is like mapping the boundaries of a forest while you are standing in the middle of it," says Churchwell. "This is very hard to do from within the galaxy."

For years, astronomers have been aware of a central feature at the heart of our Milky Way. However because of Earth's location, scientists were not sure if the feature was a stellar bar, a central ellipse, or both. Using Spitzer's dust-piercing infrared eyes to make the plane transparent, GLIMPSE astronomers brought tens of millions of previously hidden objects into view and confirmed that the central structure was indeed a bar.

"To date, this is the best evidence for a long bar in our galaxy," says Benjamin. "It's hard to argue with this data."

GLIMPSE scientists have also found that the Milky Way's bar spans roughly 27,000 light-years in length, 7,000 light-years longer than previously believed, and that the bar is oriented at about a 45-degree angle relative to a line joining the sun and the center of the galaxy.

Other GLIMPSE mapping capabilities include charting the galaxy's distribution of gas and stars.

According to Whitney, astronomers on the team have strangely found more star formation activity in the southern galactic plane, the portion of the galactic plane visible from the Earth's southern hemisphere, as in the northern galactic plane.

"We are not exactly sure what is causing this phenomenon, but we suspect that this might help locate the galaxy's spiral arms," says Whitney.

The GLIMPSE Legacy

Today, Spitzer's unprecedented infrared sensitivity sets GLIMPSE's view of the galactic plane apart from previous surveys. However, in the beginning it was this sensitivity that limited the goals of the project.

Originally this project was conceived as a broad mapping project of the entire galactic plane using both Spitzer's IRAC and Multiband Imaging Photometer (MIPS) instruments. However, concerns about the sensitivity of the instruments and their interaction with extremely bright regions in the Milky Way resulted in a more limited scope. The project was narrowed down to a somewhat smaller region of the Galaxy using only IRAC.

"None of us knew how bright the galactic plane was going to be at these wavelengths," recalls Churchwell. "But everyone was aware that this survey needed to be done."

Once GLIMPSE's data came back, astronomers throughout the community were all impressed by IRAC's clarity, sensitivity, and robust handling of bright regions. The team was so inspired by the results of the observation that they wrote another proposal to continue the survey through the galactic center with IRAC. They were recently approved and will call their new survey GLIMPSE 2.

After reviewing the GLIMPSE data and becoming familiar with MIPS' capabilities, Spitzer Scientist Dr. Sean Carey also felt compelled to propose a galactic plane survey, but this time with MIPS. He called his project MIPSGAL.

"By combining MIPS data with the information collected by GLIMPSE, astronomers will have a more precise knowledge of star formation in our own galaxy," says Carey.

According to Whitney, the GLIMPSE Legacy "is a huge data set that is just waiting to be mined. Future researchers will be using this data for years to come."

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